Low energy cord assemblies



Filed Feb. 8. 1965 l li illllllllilillllllllli.

3,342,133 LOW ENERGY CORD ASSEMBLIES Tor Alfons Strom, Nora Stad, and Gustav Allan Wetterholm, Gyttorp, Sweden, assignors to Nitroglycerin Aktiebolaget, Gyttorp, Sweden, a Swedish company Filed Feb. 8, 1965, Ser. No. 430,877 Claims priority, application Sweden, Feb. 14, 1964, 1,823/ 64 Claims. (Cl. 102-27) This invention relates to low energy cord assemblies. More particularly the invention relates to low energy cord assemblies of the type which comprises a length of cord consisting of a high velocity detonating explosive in the form of a continuous core surrounded by a tubular metal element and at least one layer of insulating material, a booster cap attached to and capping one end of said cord, a blasting cap attached to and capping the other end of the cord, and a cylinder-shaped distance member keeping said other end of the cord spaced from the contents of the blasting cap.

A low energy cord assembly of the type described above is known in the art (US. patent specification No. 3,020,844). In this known assembly, the cylinder-shaped distance or spacing member is in the form of a thin metal capsule, thus comprising a bottom portion provided with an aperture and a short, externally cylindrical portion integral therewith, this latter portion serving as the distance keeping or spacing member.

Because the metal capsule keeps the end of the detonating cord at a fixed distance from the pyrotechnical delay charge or, in a blasting cap of instantaneous action type, from the priming charge, there will be attained an improved safety against static elect-ricity. As the detonation wave can blow out a part of the delay charge in a delay blasting cap and this leads to a great spread of the burning time of various blasting caps, it is desirable also for this reason that the end of the detonating cord should be positioned at a certain distance from the delay charge. However, this again involves the drawback that the initiating strength of the fuse is reduced.

This reduced initiating strength of the fuse has certainly been compensated for by using more readily ignitable delay charges. Pyrotechnical charges consisting of 2% of boron metal and 98% red lead have been suggested for this purpose. However, such compositions are very sensitive to grinding as well as to electrostatic initiation, so that special precautionary measures are required in large-scale manufacture and use thereof.

The reduced remote initiating strength of the detonation wave from a detonating cord in a low energy cord assembly and the consequent greater spread of the burning time of the pyrotechnical combustion charge in such an assembly of delay type must be regarded as a very serious inconvenience.

This inconvenience can now be eliminated through the present invention by making the cylindrical spacing member elongated with a narrow axial bore having a crossseetion area less than 20 sq. mm, preferably between 12 and 0.75 sq. mm., corresponding to a diameter of 5 to 1 mm. if the bore is circular.

In an embodiment of the invention a part of the delay charge is placed in the bore of the cylindrical spacing member whereby is obtained that the pyrotechnical charge can, but need not necessarily be pressed in a tube in the form of a so called delay element as is conventional in delay blasting caps with electrical initiation.

In a further embodiment of the invention the delay element of a low energy cord assembly of delay type has the shape of a cylinder with axial bore, the interior of which is occupied by the pyrotechnical charge. In this atent case the spacing member preferably consists of a separate body, and it may be advantageous to provide a delay charge between the two parts. By this arrangement one is, besides, fairly independent of the ignition insensitivity of the pyrotechnical charge. The intermediate charge can always be made so sensitive that it is able to initiate even diflicultly ignitable delay charges.

In a still further embodiment of the invention the spacing member has a length lying within the limits 48 mm.

The use of a spacing member according to the invention involves the following advantages:

The operative reliability of the low energy cord assembly is improved and a less varying burning time is obtained. The composition of the delay charge can be chosen without paying very much attention to the ignition sensitivity, and this will, among other things, facilitate the manufacture, making it easier to handle the pyrotechnical charge.

The cylindrical body according to the invention serv ing as a distance or spacing member may be made of various materials such as metal, plastic, ceramics, etc.

In order ot more fully describe the present invention, reference is now made to the accompanying drawing. For the purpose of comparison, a low energy cord assembly, the spacing member of which is of conventional design has been illustrated.

FIG. 1 is a view on a larger scale of a cut-up cord or fuse in a low energy cord assembly,

FIG. 2 is a section through a low energy cord assembly with a spacing member of conventional design,

FIG. 3 shows in section one end of a low energy cord assembly with a spacing member according to an embodiment of the present invention, and

FIG. 4 shows, also in section, one end of a low energy cord assembly with a spacing member according to another embodiment of the present invention.

In FIG. 1 the numeral 1 designates a thin core of the explosive pentaerythritol tetranitrate (PETN) having a diameter of about 0.4 mm. The core is enclosed in a lead tube 2 having an outer diameter of 1.0 mm., and the lead tube is surrounded by two layers of insulating material, the layer 3 nearest to the core consisting of paper, and the outer layer 4 of plastic.

In FIG. 2 the numeral 5 designates a metal shell in a low energy co-rd assembly in which the cord is composed as shown in FIG. 1. The metal shell 5 holds an explosive charge 6, a priming explosive 7 and a pyrotechnical delay charge 8. Between the latter charge and the end of the cord or fuse is a spacing member in the form of a metal sleeve 9, the cylindrical outer wall of which rests against the inner wall of the metal shell 10 and the bottom of which is provided with an aperture through which the detonation can continue to the delay charge 8 of the blasting cap.

Attached to and capping the other end of the fuse is a so-called booster cap consisting of a metal shell 10 and an explosive charge 11 of secondary explosive only enclosed therein. The booster cap serves to transmit the detonation from a trunk line -(not shown) consisting of conventional detonating fuse by means of a clamping device 12.

FIG. 3 shows a cylindrical body 13 with a bore forming a passage from the end of the detonating core 1 to the delay charge 8 in a delay type blasting cap. The bore is partly occupied by the delay charge.

In FIG. 4 the separate cylindrical spacing body 14 with a narrower bore provides a passage from the end of the detonating core 1 to the delay charge 8. In this embodiment the latter is disposed in a cylinder 15 with a central bore, said cylinder with contents thus constituting a separate delay element.

3 The device shown in FIG. 4 has been tested with various bore diameters in the cylindrical spacing member 14 With the surprising result that the initiating strength of the detonating core increases with decreasing dimension of the bore. The following results were obtained:

Transmission distance in mm. at an initiation Diameter, mm.: g gg 1 15 3 As a delay charge was used a mixture of the following composition: 36% lead superoxide (brown lead oxide), 29% lead chromate and 35% silicon metal.

The following test series shows how the spread sinks. The tests were performed with a cylindrical spacing member having a bore diameter of 3 mm. and a bore length of 7 mm.

Spread With- Delay time desired value (1115.) Embodiment according to the invention, bore diameter 3 mm. (1113.)

Embodiment according to Fig. 2 (ms.)

Could not be carried out because the spread was too great.

What We claim is:

1. In a low energy cord assembly of the type which comprises a length of cord consisting of a high velocity detonating explosive in the form of a continuous core surrounded by a tubular metal element and at least one layer of insulating material, a booster cap attached to and capping one end of said cord, a blasting cap attached to and capping the other end of said cord, and a cylinder-shaped distance body for keeping said other end of said cord spaced from the contents of said blasting cap, the improvement that said distance body is elongated and has an axial bore, the cross section of which is less than about 20 mm. in open communication with said cord.

2. An assembly as set forth in claim 1, wherein the cross section of the axial bore is in the range of from 12 to 0.75 mmF.

3. An assembly as set forth in claim 1, wherein a delay charge is included in the blasting cap and all of said delay charge of said blasting cap is disposed in a portion of the bore of the cylinder-shaped distance body when the blasting cap is of delayed action type.

4. An assembly as set forth in claim 1, wherein a delay charge is included in the blasting cap and said delay charge is at least partly disposed in the bore of a separate cylinder-shaped body when the blasting cap is of delayed action type.

5. An assembly as set forth in claim 4, wherein the rest of the delay charge constitutes an intermediate charge between the two cylinder-shaped bodies when a part of the delay charge is disposed in the bore of a separate cylindershaped body.

References Cited UNITED STATES PATENTS 2,707,439 5/ 1955 Hamilton 10227 2,736,263 2/1956 Lewis et a1. 10227 2,796,834- 6/ 1957 McCafirey et al 10227 3,021,786 2/ 1962 Miller et al. 102-27 3,106,892 10/1963 Miller 10227 3,205,818 9/1965 Coulson l02--27 FOREIGN PATENTS 249,329 1/ 1964 Australia.

SAMUEL FEINBERG, Primary Examiner.

BENJAMIN A. BORCHELT, Examiner.

R. V. LOTTMANN, V. R. PENDEGRASS,

Assistant Examiners. 

1. IN A LOW ENERGY CORD ASSEMBLY OF THE TYPE WHICH COMPRISES A LENGTH OF CORD CONSISTING OF A HIGH VELOCITY DETONATING EXPLOSIVE IN THE FORM OF A CONTINUOUS CORE SURROUNDED BY A TUBULAR METAL ELEMENT AND AT LEAST ONE LAYER OF INSULATING MATERIAL, A BOOSTER CAP ATTACHED TO AND CAPPING ONE END OF SAID CORD, A BLASTING CAP ATTACHED TO AND CAPPING THE OTHER END OF SAID CORD, AND A CYLINDER-SHAPED DISTANCE BODY FOR KEEPING SAID OTHER END OF SAID CORD SPACED FROM THE CONTENTS OF SAID BLASTING CAP, THE IMPROVEMENT THAT SAID DISTANCE BODY IS ELONGATED AND HAS AN AXIAL BORE, THE CROSS SECTION OF WHICH IS LESS THAN ABOUT 20 MM.2, IN OPEN COMMUNICATION WITH SAID CORD. 